In industrial applications involving polymeric materials, the significance of high yields, optimized processes and cost efficiency of the materials cannot be overstated. Typically, industrial polymers are synthesized using condensation, free radical or anionic polymerization methods. When considering applications involving polymers bonded to substrate materials, there are several synthesis methods that are highly effective. The three main methods include physisorption, grafting-to and grafting-from techniques. In physisorption (FIG. 1A), ligands range from weak to strong adsorbents, and typically bind via an electrostatic or hydrogen bonding approach. In the grafting-to approach (FIG. 1B), the substrate is modified with a ligand bearing terminal functionality such that the functionality is available for post-modification. This method is typically utilized for higher yields favouring smaller molecular weight polymers. In the grafting from approach (FIG. 1C), ligands containing chain transfer agents (CTAs) are commonly utilized followed by the growth of polymer chains from the surface. Highly dense brushes can be achieved by this method, as polymer growth is dependent upon the diffusion of monomers to the growing chain end.
One application of interest for industrial polymeric materials is hydraulic fracturing (also commonly referred to as hydrofracking or fracking), which has recently seen a surge in its application to newly minted oil and gas fields. Although the technology has been known for several decades, recent improvements have made it economically feasible to extract oil and gas “horizontally” via fracking, as opposed to the more common vertical drilling. Its adoption is more pronounced in the U.S., where more than 1.1 million active oil and gas wells span across 36 states. There are, however, several concerns and difficulties in oil recovery via fracking and in particular from shale deposits at depths of several thousand feet.
Briefly, the process entails the creation of a hydraulic fracture in the geologic formation through pumping of high viscosity fracturing fluid for a short period (2-3 hours). The resulting high pressure exceeds the rock formation strength and a fracture is created. The pathways thus formed allow the oil in the fractured formations to flow into the wellbore, which enables oil recovery at high rates. Fracturing fluids typically contain a variety of additives that aid in fracture formation, delivery of proppants to the fracture zone and maintenance of good conductivity such that the networks formed do not collapse/clog. Additives include viscosifiers (high molecular weight polymers), biocides, corrosion inhibitors, crosslinkers, friction reducers, gelling agents, scale inhibitors, surfactants and pH control agents. The exact recipe for any fracturing fluid varies depending on the type and depth of the shale formation, borehole geometry, the amount of recoverable gas, etc. However, two main ingredients are a necessity: friction reducers and proppant materials.
Friction exists between the fracturing fluid and the contact surface of the steel pipe and within the water itself (as turbulence) when the fluid is pumped. High pressure can overcome the contact friction, and friction reducers are included to maintain non-turbulent flow. Friction reducers typically include a high molecular weight polyacrylamide polymer. In the presence of water, the polyacrylamide hydrates and its hydrodynamic radius increases, resulting in the prevention of turbulence in the moving water. Polyacrylamides are generally available as a dry powder, and are mixed with a mineral oil base fluid for stabilization prior to addition to the fracturing fluid. The amount of friction reducing materials typically ranges from 0.05-1% by weight of the fracturing fluid mixture.
Proppants are solid materials (generally treated sand or ceramic structures) that aid in keeping the fracture open during the oil recovery operation. The composition and geometry of the proppant can play a large role in maintaining flow of the fluids through the fractures. For instance, untreated sand can cause significant fines to be generated (due to crushing of the sand at high pressure) and may not maintain the fracture as open. There has been a shift toward chemically treated sand as proppant, especially toward formation of treated sand that can be both lightweight (to prevent settling) and high strength (to avoid being crushed).
One issue of concern in fracking is evacuation of the fracturing fluid from the established networks after oil recovery is completed. Once the pressure is released after pumping, about 60% of the fluid returns to the wellbore and can be consequently recovered and reused. However, several thousand gallons of fracturing fluid can remain in the strata following use. These retained fluids can slowly migrate to groundwater sources and/or to the surface, and may pose a significant environmental problem. For instance, polymers of the fracturing fluid (e.g., polyacrylamide friction reducers) may not degrade easily and the monomer units (acrylamide) are often classified as toxic contaminants in groundwater. Hence, it is in the interest of the industry to have solutions that directly address this issue and minimize risk in a cost-effective manner. Typically, established methods involve chemical or thermal decomposition of fracturing fluid remaining in the networks, followed by recovery into the well. However, the cleanup procedures vary between drilling companies, and effective methods of monitoring these steps are uncertain.
Thus, a need exists for materials and methods that can retain polymeric materials in desired locations during and following use. For instance, a need exists for materials and methods that can prevent the migration of polymeric components of fracturing fluids out of established fracture networks after oil recovery is completed.